High frequency power transistors for use with, e.g. power stages for base radio transmitters for mobile telephones or with high frequency radio transmitters, such as ground transmitters for digital radio and analogue TV are mainly of two types, namely bipolar and LDMOS types. A bipolar-type transistor has to be mounted on an electric insulator, whereas LDMOS-type transistors can be mounted on an underlying electrically conductive surface.
Encapsulation of LDMOS-transistors is typically effected with the aid of a ceramic electrical insulator in the form of a frame which encloses the transistor. The LDMOS-transistor is mounted on an electrically conductive so-called flange. Connected to the electrically insulating frame are connections which are used as conductors between at least one circuit board and the transistor. The ceramic insulator may be constructed from alumina, since the thermal flow will not pass through the ceramic material.
One of the problems with known techniques is that the flange must comprise CuW (a composite of copper and tungsten) in order to achieve an effective match between a linear expansion of the ceramic and linear expansion of the flange. The CuW-flange is relatively expensive and has only half the electrical conductivity of copper. Another problem is that the electrically insulating frame becomes weak and is liable to crack in the manufacture of the capsule, due to the differences in the thermal expansion between the frame and the flange, which results in a low yield and in a very expensive capsule. Furthermore, it is not possible to surface solder present day power transistor capsules.
One problem with known capsules for high frequency transistors is that they include a narrow electrically insulating ceramic frame which can crack easily in the manufacture of the capsule.
Another problem is that a so-called flange requires a special material, such as CuW, in order to obtain the best possible match between the linear expansion of the flange and the linear expansion of the ceramic frame.
Yet another problem is that CuW does not have satisfactory electrical conductivity.
The present invention addresses these problems through the medium of a capsule for at least one high frequency, high power transistor chip that includes an electrically conductive and thermally conductive flange, at least two electrically insulating substrates, at least two electrical connections, and a cover member, wherein the high power transistor chip and the electrically insulating substrates are arranged on the flange, and wherein the electrical connections are arranged on the electrically insulating substrate. The electrically insulating substrates are arranged to partially enclose the chip.
The flange may conveniently be made of copper. In a preferred embodiment, when the inventive shall be surface soldered the electrically insulating substrates are arranged on at least two side edges of the flange. The substrate is metallized from an upper side thereof, around one edge and down to an underside thereof.
The difference in linear expansion between the flange material and the substrate material may be large without danger of the substrates cracking, due to the fact that the substrates can be given a small size.
The object of the present invention is to provide a capsule that has better thermal conductivity and that is cheaper than its prior art counterpart, and which can also be surface soldered.
One advantage afforded by the invention is that copper can be used in the flange.
Another advantage is that there is no danger of the electrically insulating substrates splitting after having been hard soldered to the flange.
The invention will now be described in more detail with reference to exemplifying embodiments thereof and also with reference to the accompanying drawings.